1. Field of the Invention
This invention relates to television receiver arrangements of a kind suitable for use in a television transmission system of a character in which coded data pulses representing alpha-numeric text or other information are transmitted in a television signal in television lines where no picture signals representing normal picture information are present.
The possibility does exist, for a television transmission system of the above character, of so-called "full-channel" operation in which all of the television lines of each field are employed for transmitting coded data pulses. However, in existing television transmission systems of the above character, only television lines in the field-blanking intervals are employed for the transmission of the coded data pulses, so that such transmission is, in effect, multiplexed with the normal picture transmission. The alpha-numeric text or other information transmitted in this way can be displayed as a selectable alternative to the normal picture information by the use of a suitably adapted television receiver arrangement which can acquire the coded data pulses from the television signal. A television transmission system of the above character may be of the over-air broadcast type, or of the type which uses a cable as the transmission medium.
2. Description of the Related Art
A television transmission system of the above character is described in United Kingdom patent specification No. 1 370 535. Also, the publication "Broadcast Teletext Specification", September 1976, published jointly by the British Broadcasting Corporation, Independent Broadcasting Authority and British Radio Equipment Manufacturers' Association, gives details of a specification for a system of the above character.
In the above-identified Broadcast Teletext Specification, a quantity of message information to be displayed as an entity on a television screen is termed a page and will be so termed hereinafter. All of the pages which are available for display are transmitted in a recurrent cycle, with or without up-dating page information as appropriate. At the adapted television receiver arrangement a viewer can request any page for display, the requested page being acquired from the cyclic transmission the next time it occurs therein and stored in a display memory of the television receiver until replaced by a subsequently acquired page. A page consist of 24 display rows each having 40 character positions. The first display row (Row 0) of each page is termed a page-header and contains inter alia the page number. The transmission of each page begins with, and includes, its page-header and ends with, and excludes, the next page-header which is transmitted. Thus, it is assumed that all of the display rows which are transmitted between two successively transmitted page-headers belong to the page having the first page-header. The assumption avoids the need to include in each display row data for identifying the page to which the row belongs. However, because different groups of pages (magazines) use the same range of page numbers, so that more than one page can have the same page number, and because the pages from different magazines may have their display rows interleaved, each display row also includes data which identifies the magazine containing the page that the row belongs to.
The display rows are themselves identified by repective row numbers which are transmitted in the rows and which determine the row positions in the relevant page. This enables the rows of a page to be transmitted in any order, following the page-header for the page, and rows containing no information for display need not be transmitted. The row numbers are provided as respective 5-bit codes. Because only 24 of the 32 possible 5-bit code combinations are required for identifying the 24 display rows, it has been proposed that at least some of the 8 remaining 5-bit code combinations should be used to identify so-called "ghost" rows in each page. It is the intention that these ghost rows can be used to transmit auxiliary information which is not displayed but which is for processing by logic circuitry in the adapted television receiver arrangement to extend and/or enhance the facilities afforded by the system. Because the information contained in ghost rows is not displayed, it becomes possible to have more than one ghost row with the same row number, provided that each such row contains a sequencing bit or bits to distinguish it from another row or rows having the same row number. In principle, the total number of rows (display and ghost) which a page can contain is then limited by the maximum acceptable period that a viewer has to wait to acquire a requested page from the cyclic transmission of all the available pages.
Thus, in the present context, a display row is to be construed to mean a row containing information intended as part of the display of a page with which the display row is identified, and a ghost row is to be construed to mean a row containing information not intended as part of the display of a page with which the ghost row is identified, the information instead relating to that page in some other respect.
A television receiver arrangement of the above kind includes acquisition means for acquiring teletext information in digitally coded form, memory means for storing the acquired digital codes, and character generator means for producing from the stored digital codes, character generating video signals for the teletext display.
It is khown for the character generator means to include a character memory in which is stored character information identifying the available character shapes which the arrangement can display. The character information is selectively addressed in accordance with the stored digital codes and the information read-out is used to produce the character generating video signals for the teletext display.
The teletext transmissions in the United Kingdom are all in one language, namely English, and the character generator employed for these transmissions produces English language characters from the acquired digital codes. If such transmissions were provided in a country whose national language used additional characters to those of the English language, or characters which differ to English language characters, it would be a simple matter to provide for these transmissions a character generator having a character memory for the alphabet of the country concerned. Such a simple solution however does not fit the needs of a country which can receive teletext transmissions from neighbouring countries having languages whose alphabets differ from that of its national alphabet. Such a situation can be found at a number of places on the Continent of Europe where in addition to the basic Latin alphabet different countries employ different accented characters. One way of overcoming this problem and allowing reception of different teletext transmissions in different languages is proposd in B.B.C. Engineering, December 1977, page 2, in the article Alphabets for CEEFAX where it is suggested that a character generator could have a character memory providing different character sets which are for selection according to the language transmitted by using control bits on each page to select the correct character set for that page. The control bits which it is suggested could be used are three (C12, C13 and C14) contained in the Page-Header (Row 0) which are at present unallocated.
A study of the Centre Commun d' Etudes de Telecommunications et Television (C.C.E.T.T.) Report TSA/T/14/77, "Problems Linguistiques du Teletexte. Etudes des langues Europeennes utilisant l'alphabet Latin--Rapport final", Rennes, July 1977 reveals that the character requirements of all the major European languages can be met with a number of character sets having control and character codes according to International Standard ISO 646. These character sets differ only in the use of a few (11) national use character options.
Thus for teletext transmissions in the major European languages the coding of the International Standard ISO 646 could be adopted and for each character set it is possible to use a character memory dedicated to that character set. Where a teletext television receiver arrangement is used in a location where it is possible only to reccive teletext transmissions in one language then only the character memory suitable for decoding the coded characters in that language is required. However, where the teletext receiver arrangement is to be used in a location where it possible and desired to receive teletext transmission in different languages there could be employed an appropriate number of character memories corresponding to the number of character sets to be decoded. As many of the characters between the languages are the same the separate character memories could be formed from a single composite character memory capable of having the (eleven) national character options decoded according to the language.
However, the selection of a particular character set, from the options which are available, by means of control bits contained in the page header (Row 0), has the limitation that it is not possible to mix languages on a single page as identified by the page header. Another more serious limitation occurs in connection with a facility proposed for a next generation of teletext decoders which are micro-processor controlled, whereby locally-generated status messages can be displayed under the control of the microprocessor on an additional row either alone or with a displayed page which contains broadcast rows. Such a status message should be in a given language which should not change even though the language of broadcast pages may vary. Thus, this status message facility can require a mixed language page display which cannot be achieved using the control bits in the page header.